CB-8758 [wp8]: UnauthorizedAccessException on hide()
CB-8758 [wp8]: UnauthorizedAccessException on hide()

When calling hide() from JS, the attempt to access Popup could happen on
a background thread, which ends up throwing an
UnauthorizedAccessException about cross-thread access.

To address this, it's best to check the popup's state from within the UI
thread.

Fixes https://issues.apache.org/jira/browse/CB-8758

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